Steam-cracking of light paraffins (ethane, propane and butane, obtained mainly by extraction from various natural gas sources) and of naphthas and other heavier petroleum cuts, produces:    i) primarily ethylene and propylene;    ii) secondarily, depending on the feedstock employed, a C4 cut rich in butadienes and a C5+ cut with a high content of aromatics, particularly benzene; and thirdly    iii) hydrogen.
The feedstocks of choice are ethane and LPG for the U.S.A. and naphthas and gas oils for Europe. However, in recent years, the situation has dramatically changed with the U.S.A. moving towards the utilization of heavier hydrocarbon feedstocks. It is worth noting that steam cracking is one of the core processes in the petrochemical industry with a worldwide production of ca. 100 million metric tons/year of ethylene and propylene.
Steam cracking is a thermal cracking reaction performed at high temperatures and in the presence of steam, a diluent which is coifed with the hydrocarbon stream. The reaction temperature ranges from 700° C. to 900° C. according to the type of feedstock treated (the longer the hydrocarbon molecular structure, the lower the temperature of cracking), while the residence time ranges from a few seconds to a fraction of second.
Steam cracking is a well-established technology. However, it suffers from many drawbacks:    i) lack of flexibility in the product selectivity, mostly in the yield of propylene, which needs to be increased in order to respond to the increasing demand of the market.    ii) significant production of fuel oil which contains heavy hydrocarbons such as heavy alkylaromatics and even polyalkylaromatics. It is known that the latter products are precursors of <<coke>>. Coking is a serious problem in the steam cracking technology, which decreases the energy efficiency and requires non-easy decoking procedures for reactors.    iii) in order to achieve a high conversion, a high operational severity is currently used which consists mainly of using high reaction temperatures and the recycle of some gaseous paraffinic products.
A process aiming at upgrading the products of propane steam cracking was developed more than twelve years ago [1]. This process consisted of adding a small catalytic reactor to the conventional steam cracker of propane. The catalyst used was based on the ZSM5 zeolite, modified with Al and Cr [2]. Significant increases in the yield of ethylene and aromatics were obtained.
A new process, consisting of using two reactors in sequence, was recently developed [3,4]. The first reactor (I) contains a mildly active but robust catalyst, and the second reactor (II) is loaded with a ZSM5 zeolite based catalyst, preferably of the hybrid configuration. Variations of the temperature of reactor I, and the textural properties, and/or the surface composition of the catalyst of reactor (II), are used to increase the conversion and to vary the product distribution, namely the ethylene/propylene ratio.
Although such a process is of great industrial interest, the use of two reactors, which may be heated separately, represents a significant challenge in terms of technology and investment. Therefore, to change the previous two-reaction-zone technology into a one-reaction-zone technology, hybrid catalysts capable of expressing several functions [5] have been proposed. Such hybrid catalysts contain a microporous component such as a ZSM-5 zeolite or a pentasil-type silicalite, a mesoporous silica-alumina co-catalyst into which is incorporated aluminum oxide, chromium oxide or a mixture of aluminum and chromium oxides, and an inorganic binder such as bentonite clay.
There thus remains a need to develop hybrid catalysts useful in one-reaction zone technology, for the deep catalytic cracking of petroleum naphthas and other hydrocarbon feedstocks.
The present invention seeks to meet these and other needs.
The present invention refers to a number of documents, the content of which is herein incorporated by reference in their entirety.